On the Globular Cluster Color Distributions

Abstract

The bimodal color distribution of globular clusters (GCs) is an outstanding discovery from observations of early-type galaxies [1,2]. Yoon, Yi, & Lee have recently presented a new theoretical metallicity–color relationship that has a significant inflection and showed that such a relation can produce bimodal color distribution of an old GC system even when the intrinsic metallicity distribution is unimodal [3]. We have applied this hypothesis to (1) the considerable variation in color histogram morphologies from color to color, and (2) the broad unimodal color distributions of GCs found in some galaxies. Our population model suggests the different shapes of the [Fe/H]–color relations depending on the color used. As a result, the [Fe/H] distribution of a GC system can be transformed into color distributions that significantly vary from color to color, consistent with the currently available data (Fig. 1). On the other hand, the simulation shows that the presence of a metal-rich, young GC population in addition to the old population may be responsible for the broad unimodal color distributions (Fig. 2). The results may be taken as a further support for the Yoon, Yi, & Lee hypothesis.